1. Field of the Invention
The present invention generally relates to an image forming apparatus, such as a laser printer or a copier, and more particularly, to an image forming apparatus for duplex printing having a paper conveyance guide using both surfaces thereof as paper feeding surfaces.
2. Description of the Related Art
Generally, an image forming apparatus for printing on both sides of a paper sheet (duplex printing), for example, a laser printer 1, includes a paper feeding cassette 10 having a stack of paper sheets 15 therein, a pickup roller 20 mounted on an upper side of the paper feeding cassette 10 to pick up the paper sheet 15, a paper conveyance guide 22 defining a first paper conveying path that guides the paper sheet 15 picked up by the pickup roller 20, a developing unit 30 that develops a visible toner image on a first side of the paper sheet 15 being conveyed through the paper conveyance guide 22, a fixing unit that fixes the toner image on the first side of the paper sheet 15 passing through the developing unit 30, a conveying roller 50 that conveys the paper sheet 15 that has passed through the fixing unit, a discharge roller 60 arranged on an upper side of the conveying roller 50 to discharge the paper sheet 15, a reverse paper conveyance guide 62 defining a second paper conveying path that guides the paper sheet 15, which is discharged through the discharge roller 60, to re-enter the paper conveyance guide 22 for the duplex printing so as to have a second side of the paper sheet 15 printed, and a first and a second reverse conveying rollers 70 and 90 that reverse-convey the paper sheet 15 through the reverse paper conveyance guide 62a as shown in FIG. 1.
The developing unit 30 includes a photosensitive drum 32 that receives light signals, such as a laser beam, from a laser scanning unit (LSU) 36 in accordance with an image signal, thereby forming an electrostatic latent image thereon, and a developing roller (not shown) that attaches toner particles onto the electrostatic latent image of the photosensitive drum 32, thereby forming the toner image.
A transfer roller 34 is disposed below the photosensitive drum 32 of the developing unit 30 to transfer the toner image from the photosensitive drum 32 to the paper sheet 15.
The fixing unit includes a heating roller 41 and a fixing roller 40 fixing the toner image on the paper sheet 15 with heat and pressure.
Formed between the paper conveyance guide 22 and the paper reverse conveyance guide 62, at which the discharging roller 60 is disposed, is a paper reverse guide 42 guiding the paper sheet 15 to the paper reverse conveyance guide 62. The paper reverse guide 42 is formed as an upright arm flexibly hinged between the conveyance guide 22 and the paper reverse conveyance guide 62. On the paper reverse guide 42 is formed a paper sensor (not shown) that senses the paper sheet 15 being conveyed.
The duplex printing by the laser printer 1 constructed as above will be described below.
First, each paper sheet 15 is picked up by the pickup roller 20 from the paper stack of the paper feeding cassette 10, and then conveyed to the developing unit 30 along the paper conveyance guide 22.
By a laser beam emitted from the LSU 36 in accordance with the image signal, the electrostatic latent image is formed on the photosensitive drum 32 of the developing unit 30, and the electrostatic latent image formed on the photosensitive drum 32 is developed into the toner image by the developing roller that attaches the toner particles onto the electrostatic latent image.
The toner image formed on the photosensitive drum 32 is transferred onto the first side, e.g., an upper side, of the paper sheet 15 while the paper sheet 15 is conveyed to the developing unit 30 along the paper conveyance guide 22, and then fixed on the upper side of the paper sheet 15 by the heating roller 41 and the fixing roller 40 of the fixing unit.
After passing through the heating roller 41 and the fixing roller 40, the paper sheet 15 is advanced to the discharging roller 60 by the conveying roller 50. By a rotational force of the conveying roller 50, the paper sheet 15 pushes the paper reverse guide 42 from a dotted line position to a solid line position of FIG. 1. Accordingly, the paper sensor detects that the paper sheet 15 is passing forward the discharging roller 60.
When the printing is performed on the first side of the paper sheet 15 only, i.e., when it is simplex printing, the paper sheet 15 is released onto a paper panel 59 via the discharging roller 60.
In the duplex printing, however, the discharging roller 60 is reverse-driven by a separate forward/reverse rotation control unit (not shown) in accordance with a signal or signals from the paper sensor which generates the signal after a rear tip end of the paper sheet 15 passes through the paper reverse guide 42. As a result, the paper sheet 15 is not released out, but re-entered into the reverse paper conveyance guide 62. Since the paper reverse guide 42 is in the dotted line position, i.e., in an original position due to its recovery force, the paper sheet 15 can be easily re-entered into the reverse paper conveyance guide 62 by the paper reverse guide 42.
When the paper sheet 15 reaches the reverse paper conveyance guide 62, the paper sheet 15 is conveyed to an aligning roller 80 by the first reverse conveying roller 70. The aligning roller 80 is in a tilted posture at an angle about 5°, and thus, a leading tip end of the paper sheet 15 is pushed by the aligning roller 80 into an alignment with the second paper convey path.
After being aligned by the aligning roller 80, the paper sheet 15 is passed through the second reverse conveying roller 90 and conveyed back to the paper conveyance guide 22. Then, after the printing is performed on the second side of the paper sheet 15 in the same way as described above, the paper sheet 15 is released out onto the external paper panel 59.
In the conventional laser printer 1 constructed as above, the paper conveyance guide 22 and the reverse paper conveyance guide 62 are arranged separately from each other. Further, the conventional laser printer 1 requires a number of parts for defining a space for the first and the second paper conveying paths, such as three upper guide plates and four lower guide plates. As a result, a manufacturing cost increases. Further, since an assembling process requires fasteners, such as screws, which are rather complex to assemble, the assembling process takes a lengthy time which deteriorates productivity.
Further, according to the conventional laser printer 1, the first and second paper conveying paths are formed all across the printer 1 in a lengthwise direction. Accordingly, it also takes time in the duplex printing, deteriorating a printing efficiency (both a duplex printing speed and a simplex printing speed). Also, it has a high possibility of having problems, such as a paper jam, a paper skew, or the like.
As the paper conveying path is long, it needs a considerable number of rollers to convey the paper sheet 15 to the paper conveyance guide 22. For example, the reverse paper conveyance guide 62 needs at least three types of rollers, i.e., the first reverse conveying roller 70, the aligning roller 80 and the second reverse conveying roller 90.
Not only are these rollers 70, 80, 90 quite complex to assemble, but the assembling process also requires a number of parts, such as a shaft fixing ring, a power transmitting belt, a pulley for belt, bushing, etc. Accordingly, the assembling process becomes complex, requiring a longer assembling time and a higher cost. Additionally, there is a high possibility of having defects.
More specifically, driving power is transmitted to the first reverse conveying roller 70 and the aligning roller 80 by an idle gear assembly 95 which is employed as a power transmitting unit shown in FIG. 2. The idle gear assembly 95 includes a first idle gear 97a rotatably secured to a first boss 96a by a first shaft fixing ring 99a to be meshed with a gear portion (not shown) formed at an end of a shaft of the first reverse conveying roller 70 while the first boss 96a is secured at the reverse paper conveyance guide 62, a second idle gear 97b rotatably secured to a second boss 96b by a second shaft fixing ring 99b to be meshed with the first idle gear 97a while the second boss 96b is secured at the reverse paper conveyance guide 62, and a third idle gear 97c rotatably secured to a third boss 96c by a third shaft fixing ring 99c to be meshed with the gear portion (not shown) of the aligning roller 80 and the second idle gear 97b while the third boss 96c is secured at the reverse paper conveyance guide 62.
Since the aligning roller 80 and the second reverse conveying roller 90 are spaced apart from each other by a predetermined distance, as shown in FIG. 3, the driving power needs to be transmitted through a belt pulley 93 secured to a side of the shaft 91 of the second reverse conveying roller 90 by an E-shaped ring 94, a belt pulley (not shown) formed corresponding to the shaft of the aligning roller 80 and a power transmitting belt 93a connecting such belt pulleys.
Further, each of the rollers 70, 80, 90 is fabricated through a rather complex process. For example, the second reverse conveying roller 90 has the shaft 91 formed separately from the gear portion and the belt pulley 93, while the roller portion 92 is formed on the shaft 91 by molding.
Further, since the rollers 70, 80, 90 are secured to the reverse paper conveyance guide 62 using the bushings 98, 98′, the assembling process requires complicated additional work that takes much longer time.
As a result, the conventional laser printer 1 has the first and second long paper conveying paths for duplex printing, requiring many parts and processes, and thus has the problems of a manufacturing cost increase, printing efficiency deterioration, a frequent paper jam, and productivity deterioration.